Improvement of anti-corrosion performance of an epoxy coating using hybrid UiO-66-NH2/carbon nanotubes nanocomposite

In this study, a porous nanocontainer from UiO-66-NH2/CNTs nanocomposite with an excellent barrier characteristics was constructed through amine-functionalized Zr-based metal organic framework. The characterization of the prepared nano-materials were performed using different analyses such as FTIR, XRD, SEM, EDS, TEM, and BET and the results proved the successful synthesize of UiO-66-NH2/CNTs nanocomposite. The corrosion protection performance of the coated panels was investigated by electrochemical impedance spectroscopy (EIS), salt spray, and contact angle measurement. The EIS results revealed that unmodified and UiO-66-NH2 containing coating in 3.5 wt.% NaCl electrolyte were failed after 45 days but the corrosion was negligible in UiO-66-NH2/CNTs coating due to high pore resistance values even after 45 days. Salt spray and contact angle measurements confirmed that UiO-66-NH2/CNTs containing coating acts as an efficient barrier against wet saline environment even at long exposure times. This is attributed to uniform dispersion in the epoxy matrix and formation of a uniform nanocomposite coating.

As a promising material for delaying ions and water molecules from reaching to metals' surface, polymeric coatings act like a physical barrier. The epoxy resins could be considered as a group of distinguished thermosetting polymers with outstanding features of excellent resistance to moisture, remarkable resistance to solvents, excellent mechanical and thermal properties, and great adhesion properties to various surfaces of both metals and non-metals. Due to its excellent properties, this material has been utilized in a wide range of industries including aircraft construction, car industries, and petroleum industries; Nevertheless, suffering from some salient weaknesses such as poor crack deflection performance, and brittleness, it couldn't be used in many applications 1,2 . Therefore, finding a suitable alternative for epoxy resin among organic coatings is challenging as these materials are not water-proof and totally perfect. Resultantly, in a corrosive environment, metals could not be preserved for long period. Formation of defects (in micro-scales) in the structure of coatings is almost inevitable. The situation gets worse when the material is designed for outdoor application where it experiences harsh and uncontrolled conditions. Bypassing the coating material, the structural defects result in the corrosion of metal. So as to prevent such a kind of problem, a new generation of coating with the ability of anti-corrosion has been constructed. The protective ability could be improved by addition of nano/micro additives to the structure of epoxy-based coatings. Various investigations have dealt with the construction of high quality nanocomposites with excellent mechanical and thermal features [3][4][5] . A wide variety of nanofillers are investigated and chosen for enhancement of the coatings' protectiveness. For example some of carbon-based nanofillers are: carbon nanotubes 6,7 , graphene and graphene oxide 8,9 , inorganic nanomaterials such as LDH 10 , fullerene 11 , and halloysite 12 and clay 13 , etc. In recent decades, a group of porous materials with a 3D structure have been widely investigated and developed significantly. Since introduced as materials with nanopore structure, modifiable properties by replacing its ligands, and astonishingly large surface areas, metal-organic frameworks (MOFs) have attracted many attentions [14][15][16][17][18][19] . Additionally, these porous materials have been contemplated as promising candidates for fabrication of anti-corrosion coatings with barrier capability. Furthermore, not only the resulted coatings will prevent corrosion properly, but also it has been observed that they are relatively impermeable. Unfortunately, rarely reports could be found covering the application of MOFs in the fabrication of anti-corrosion coatings or their application as a protective layer [20][21][22][23][24][25][26][27][28][29][30][31]  Characterization and techniques. The fingerprint and structural functional groups of the materials were determined using Fourier transmitter infrared spectroscopy (FTIR, Perkin-Elmer, Spectrum One, USA). X-ray diffraction (XRD, PANalytical, Netherlands) was employed to distinguish crystal structure and the data was collected by Bruker D8 advance diffractometer utilizing Cu/Kα radiation source and voltage 40 kV, 40 mA, and 1.54056 Å. The sample was scanned between 5 and 65.95 degrees 2θ, in step sizes of 0.01° at a rate of 0.01° 2θ/second. The scanning electron microscopy (SEM) (LEO 1455VP, Oxford, UK) and transmission electron microscopy (TEM) (Zeiss-EM10C-100 kV, Germany) were run to find out the crystal size and the surface morphology of all samples. The specific surface area of the synthesized MOF and nanocomposite were identified using Brunauer-Emmett-Teller (BET) measurements (BELSORP-mini II, BEL, Osaka, Japan) by N 2 adsorption-desorption isotherm at 77 K.
Electrochemical impedance spectroscopy (EIS). In order to study the electrochemical properties, a potentiostat/galvanostat (Autolab, PGSTAT 302 N) was used. Coated samples (1 cm 2 exposed area), saturated calomel electrode (SCE) and a platinum electrode were employed as working electrode (WE), counter electrode and reference electrode, respectively. Impedance measurements were performed by an AC signal with the amplitude of 20 mV peak to peak at the open circuit potential in the range of 100 kHz to 10 mHz frequency. The coated samples were immersed in 3.5% NaCl solution and EIS measurements were performed over time. The Nova (version 1.6) and Z-View2 softwares were used to data measurements and fit the EIS data via the electrical equivalent circuits (EEC), respectively. All EIS experiments were repeated for three times under the same conditions to ensure that the results are reproducible.
Salt spray test. Salt spray exposure was carried out on the steel samples coated with epoxy coating. The test was done on the scratched samples for 500 h in salt spray cabinet according to ASTM B117. The NaCl concentration of salt fog was 5 wt.%. All samples were scratched with a dimension of 3 cm × 2 mm, created using a surgery knife.

Contact angle measurements.
Contact angles were measured using a home-made system at room temperature to investigate the static contact angles of distilled water droplet (5 µL) on the different coating. Water droplets were carefully placed onto the surfaces of the samples and the shape of droplet was recorded by canon digital camera. The homemade apparatus has been calibrate with standard references before measurements. The contact angle was determined from the average of five measurements at various positions on the samples surface.

Results and discussion
Characterization of the prepared materials. Different characteristic analyses were investigated to confirm the successful preparation of the nanomaterials. The scanning electron microscopy (SEM) analysis was employed to evaluate the surface morphology and structural characteristics of the synthesized nanoparticles. Figure 1a shows the carbon nanotubes after acid-treatment, which possess about 10-35 nm diameter. Figure 1b illustrates the pure UiO-66-NH 2 crystals (50-400 nm) with a uniform and octahedral shape. The morphology of hybrid UiO-66-NH 2 /CNTs nanocomposite reveals a three-dimensional interlinked network (Fig. 1c). It can be clearly seen that the CNTs and UiO-66-NH 2 nanoparticles retain their tubular and octahedral structures after hybridization. Also, EDS mapping represents the elemental scattering of C, O, N, and Zr in the UiO-66-NH 2 / CNTs framework, verifying the successful fabrication of the nanocomposite. Further investigation was performed using transmission electron microscopy (TEM) analysis to show the micro-cavities of the nanoparticles  Figure 2a shows the UiO-66-NH 2 particles, which confirms the octagonal structure with moderate aggregation. The TEM image of UiO-66-NH 2 /CNTs hybrid nanocomposite with two different magnifications, shown in Fig. 2b, c, indicates the growth of UiO-66-NH 2 crystals on the outer wall of CNTs as clusters of grape. Also, the electron scattering pattern (Fig. 2d) for a selected area, clearly shows the bright spots in the form of a circular ring, which determines the frequency of the synthesized crystalline lattice structure. Electron diffraction is performed on a TEM by using the magnetic lenses of the beam column to focus the beam down to a point that can be aimed at a single particle or edge of a larger crystal. The result is a black image with points of light where the crystal structure is causing the beam to scatter. With the main beam blocked in the center of the image, these points of light or rings in the case of an amorphous or polycrystalline material can be used to calculate information on the crystal structure of the sample. The FTIR spectra of CNTs, UiO-66-NH 2 and UiO-66-NH 2 /CNTs hybrid nanocomposite are shown in Fig. 3a. The UiO-66-NH 2 spectrum illustrates three specific peaks (marked with small arrows) at 1400, 1580, and 1655 cm −1 . The FTIR peak at around 1400 cm −1 attributed to symmetric vibration of carboxyl group. The carbonyl group vibration band appeared at 1655 cm −1 and the peak at 1580 cm −1 ascribed to aromatic C=C stretching vibration. In addition, two typical peaks located at 788 and 1260 cm −1 attribute to N-H and C-N stretching band and confirm the presence of -NH 2 group in the UiO-66-NH 2 structure. Furthermore, two characteristic peaks at 3518 and 3337 cm −1 are derived from the asymmetric and symmetric vibrational bands of the primary amine group, respectively 67 . Some peaks existing in the range of 400-700 cm −1 are belonged to in-plane and out-of-plane -COO groups ' vibrations 68,69 . The FTIR spectra of UiO-66-NH 2 /CNTs nanocomposite shows that the main bands are relatively remained with no variations in comparison with the pure UiO-66-NH 2 . However, some small changes can be observed at some peaks such as 574 cm −1 , 1064 cm −1 (highlighted with orange color) and 1253 cm −1 (highlighted with green color), which is due to the interactions between CNTs and the MOF crystals (e.g. π-π interaction) and introducing functional groups brought with the acid-treated CNTs.
The phase structures of the prepared materials were further analyzed using XRD analysis for confirming the crystallinity. As shown in Fig. 3b, the characteristic peak with low intensity and a relative broad peak are observed at 23° and 43° in CNTs pattern which indicate amorphous crystal form and refer to (002) and (100) planes, respectively 70 . The XRD pattern of the UiO-66-NH 2 sample shows a great agreement with the patterns reported in literature (Fig. S2) 68,71 . The specific peaks appeared at 2θ = 7.3°, 8.5° and 25.6° attribute to the (110), (200), and (600) planes, respectively 65 . According to the XRD pattern obtained for the UiO-66-NH 2 /CNTs nanocomposite, quite similar peaks to that of the UiO-66-NH 2 can be seen with no significant displacement, confirming that the UiO-66-NH 2 nanoparticles have been successfully loaded on the CNTs by employing the solvothermal preparation method. N 2 adsorption-desorption isotherms of the nanomaterials were also investigated to measure the BET surface area, as shown in Fig. 3c. The obtained specific surface areas comprise 65.35, 1113, and 1064.4 m 2 /g for CNTs, UiO-66-NH 2 , and UiO-66-NH 2 /CNTs, respectively (Table S1). All samples possess the type H 3 hysteresis loop and reveal type I isotherm according to the IUPAC suggesting microporous structure of the materials (Fig. S3). Additionally, the pore size distribution of different samples were shown in Fig. S4. The uniform pore structure confirmed that the samples had well-defined mesopores. Beside the SEM analysis, the elemental composition of the UiO-66-NH 2 /CNTs sample was identified using energy dispersive spectroscopy (EDS) technique. Figure 3d represents the result of the EDS analysis which indicates the presence of carbon, zirconium, oxygen and nitrogen as the most dominant elements in the sample. Four prepared samples (See "Characterization and techniques" section) were exposed to the experimental electrolyte and EIS data were extracted in a time-dependence manner to pursue the key changes in the performance of additive-free and composite coatings over time. The electrolyte diffusion is occurred through the defects and low cross-linked regions into the organic coatings. When the electrolyte reach to the steel substrate corrosion will initiates and spreads in the coatingmetal interface. So, the system degradation is inevitable at prolonged immersion times. The Bode plots, as the most beneficial output of EIS technique, are reported in Fig. 4. The key factors in the EIS study of protective organic coatings are maximum impedance magnitude (Z max ), the part of Bode phase plot that is in the maximum value (θ -90 ), number of time-constants (N tc ), and breakpoint frequency (f br ). These factors (except N tc ) are depicted in Fig. 5. As can be seen in Fig. 4, all samples showed one time-constant except unmodified/epoxy and UiO-66-NH 2 /epoxy samples after 45 days. The appearance of second time-constant could be a sign of double-layer formation in the organic-inorganic interaction face 72 . The double-layer creation is due to water uptake and consequent electrolyte accumulation in some areas of the metal-coating interface. So, the corrosion process initiated in the unmodified-epoxy and UiO-66-NH 2 /epoxy samples with longer exposures (45 days). However, the corrosion rate was not severe (See θ -90 description). Samples with one time-constant were in the immune region, although the corrosion rate could not be assumed to be zero. The resistance behavior observed in the bode-module plots of all samples showed that water penetrates to the metal surface through diffusion paths but the corrosion was negligible in CNTs/epoxy and composite samples due to high pore resistance values even after 45 days (See Z max description). So, N tc values and resistive-capacitance behavior of EIS plots confirmed that CNTs/epoxy and composite samples have relatively good corrosion resistance and two other samples were failed after 45 days. www.nature.com/scientificreports/ As presented in Fig. 5a, the UiO-66-NH 2 /CNTs/epoxy composite and UiO-66-NH 2 /epoxy samples showed the most Z max values after 15 days. Also, the Z max value of UiO-66-NH 2 /epoxy is higher than the unmodified/ epoxy and CNTs/epoxy after 15 days. The observed better performance is due to the better barrier performance of UiO-66-NH 2 /epoxy. The UiO-66-NH 2 are dispersed into the coating and reinforce it against electrolyte diffusion. However, as mentioned in the introduction section, the CNTs show improper dispersion in the epoxy coating and show weaker behavior at short exposure times. However, Z max followed a considerable decreasing trend for UiO-66-NH 2 /epoxy and reduced below 6 MΩ.cm 2 after 45 days. Thus, organic-inorganic composite based on UiO-66-NH 2 and epoxy were not good candidates for corrosion protection of metals without any modification. The CNTs/epoxy exhibited more acceptable protective behavior related to the unmodified/epoxy and UiO-66-NH 2 /epoxy samples at longer exposures. Although, the best performance was recorded for composite sample which Z max was about 1000 MΩ.cm 2 even after 45 days 73 .
According to Fig. 5b, the θ -90 values of unmodified/epoxy and UiO-66-NH 2 /epoxy samples considerably decreased over time and reached close to zero after 45 days. Despite the remarkable reduction in the θ -90 values, the phase angle at high-frequency region still was near -90 degree, indicating that unmodified/epoxy and UiO-66-NH 2 /epoxy were not fully destructed as per mentioned in N tc description part. The recorded results for CNTs/ epoxy and composite samples were different and their ascending trend was not sharp over time. The θ -90 is a good criterion for estimating the activated interface area on the metal surface 74 . The higher θ -90 values, the lower active area. So, in accordance with the N tc results, the most θ -90 values were observed for composite sample confirming the best performance against electrolyte penetration into the interfacial face. Finally, the f br criteria was used to peruse the coating ability in contact with high saline solution over time. f br is an efficient criteria to evaluate the electrochemical active area in the metal/electrolyte interface 75,76 . As observed in the Fig. 5c, the lowest log In the second part of this section, all EIS plots were fitted with appropriate equivalent circuits (ECs) and the collected data were used to better understanding the anti-corrosion behavior of composite sample. The extracted results are reported in Table 1. Also, two different ECs are illustrated in Fig. 6. The constant phase element parameter Q was used instead of ideal capacitor (C) in Fig. 6 due to the surface heterogeneity. Composite coating resistance (R pore ) and constant phase element parameters (Q c and n c ) as well as double layer characteristics including charge transfer resistance (R ct ) and constant phase element parameters (Q dl and n dl ) were recorded versus exposure time to find out coating-electrolyte-metal interactions. The ideal coating (C c ) and double layer (C dl ) capacitance were calculated as per mentioned elsewhere 77,78 .
As shown in Table 1, the relative growing trend in C c values is detected in all samples versus immersion time. Water uptake is an inevitable phenomena in organic coating exposed to the aqueous electrolytes. So, the C c is increased by water penetration into the composite coatings. Also, the lowest C c values were recorded for composite sample confirming the minimum water uptake among all samples. So, although the C c could be affected by coating swelling at prolonged exposure times, it could be concluded that the barrier action of epoxy coating was reinforced by UiO-66-NH 2 /CNTs nanocomposite additive even after relatively long exposures to high saline media 79 . However, further investigation was needed for finding the cause of lower water uptake and better corrosion protection in the composite sample. So, the hydrophobicity of the samples were studied by using of water contact angle measurement. . On the other hand, the composite and UiO-66-NH 2 /epoxy samples show hydrophobic characteristics and a larger water contact angle 81 . So, it seems reasonable to accept that the hydrophobicity nature of composite effectively decreased the adsorption of water onto the surface; and composite and CNTs/epoxy coatings show good barrier properties in wet environments. However, the protection behavior of composite sample is considerably better than CNTs/epoxy, especially  www.nature.com/scientificreports/ at longer exposures. Another factor that efficiently affects the barrier behavior of a composite coating is additive dispersion in the composite matrix. The addition of CNTs into the epoxy resin without any surface treatment is difficult due to the nanotubes aggregation 82,83 . The non-uniform dispersion of CNTs creates weak regions in the final cured epoxy system which are sensitive to wet conditions 83 . The water uptake and initiation of interfacial interactions are inevitable at prolonged exposures in the presence of these weak regions. On the other hand, UiO-66-NH 2 /CNTs additive is appropriately dispersed in the epoxy media and yield a uniform nanocomposite coating acts as efficient barrier against wet saline environment even at long exposures.
Salt spray exposure. The salt spray test was carried out at different exposure times on the coating with artificial defect to reveal the inhibitive performance of UiO-66-NH 2 /epoxy, CNTs/epoxy and composite samples. The visual performances of the samples are showed in Fig. 8. Corrosive media, such as chloride ions and oxygen diffuse the steel/coating interface through the micro porosities presented in the coating structure or along the scratch. When the corrosive ions penetrate the metal/coating interface, the corrosion reactions happen. So, adhesion bonds destruction and coating delamination occur due to the cathodic reactions and accumulation of corrosion product, respectively. According to Fig. 8, the corrosion reaction takes place in the scratch region of only UiO-66-NH 2 /epoxy after 250 h. By increasing exposure time up to 500 h, corrosion process is intensified and consequent products are speared around the scratch on all samples. Also, UiO-66-NH 2 /epoxy is the only sample shows coating delamination after 500 h. However, blisters are not appeared in all samples. Comparing the intensity of damaged area on scribe region indicates that addition of UiO-66-NH 2 /CNTs nanocomposite to the epoxy coating caused significant decrease of the corrosion. The smaller area of corrosion damage and better visual performance of composite sample around the scratch could be attributed to the better barrier perfor-  Protection mechanism. As presented in previous section, the protective behavior of prepared samples could be categorized into the two mechanisms. Addition of CNTs in the form of UiO-66-NH 2 /CNTs nanoadditive improve their dispersion into the epoxy matrix. So, the regions contained agglomerated additives (due to the CNTs entanglement) is not formed in the coating and the electrolyte diffusion is reduced, especially at shorter immersion times. This behavior is also observed in the UiO-66-NH 2 /epoxy after 15 days exposure due to the relatively good dispersion of UiO-66-NH 2 additive. Another mechanism is based on the coating hydrophilicity. The water uptake and consequent corrosion of the substrate is proportional to the coating hydrophilic behavior. Despite acceptable anti-corrosion performance of the UiO-66-NH 2 /epoxy at short exposure times, its protectiveness shows considerable reduction after 30 days. However, other samples show lower reduction in Z max value between 15 and 30 days. On the other hand, composite sample with the most contact angle provide the best protective behavior, even after 45 days. So, it can be concluded that lower hydrophilicity and more compactness are the two reasons for better corrosion protection action of composite coating in the presence of UiO-66-NH 2 / CNTs nano-additive.

Conclusions
Fabrication of nanocomposite coating with uniform structure and hydrophobic nature related to non-modified epoxy coating is a key solution for designing an advanced protective organic coating. Accordingly, the UiO-66-NH 2 /CNTs nanocomposite was synthesized and epoxy-based coating was performed using this nanocomposite additive. FTIR, XRD, SEM, EDS, TEM and BET characteristic analyses were performed to confirm the synthesizing of UiO-66-NH 2 /CNTs nanoparticles. The outcomes showed that the UiO-66-NH 2 /CNTs nanoparticles have been successfully synthesized. The most maximum impedance magnitude (Z max ) and the part of Bode phase plot that is in the maximum value (θ -90 ) and the lowest value of breakpoint frequency (f br ) for UiO-66-NH 2 /CNTs/epoxy coating after long exposures in 3.5% NaCl solution indicated better performance of composite coating. The contact angle measurements represented UiO-66-NH 2 /CNTs additive increase coating hydrophobicity related to the UiO-66-NH 2 /epoxy and CNTs/epoxy coatings. According to the salt spray exposure, the smaller area of corrosion damage and visual performance around the scribe region confirmed that www.nature.com/scientificreports/ the addition of UiO-66-NH 2 /CNTs nanocomposite to the epoxy coating could effectively enhance corrosion properties. So, it can be concluded that the main mechanisms for better corrosion protection behavior of the composite sample are the lower coating hydrophilic nature (leading to lower water uptake) and better additives uniformity resulting in better barrier performance.